Volume control system



Feb. 19, 1935. Q wHELAN 1,992,021

VOLUME CONTROL sirs'jrmm Filed septa-15, 1931 2 Sheets-Sheet 1 To voice signal source.

Inventor: Dow O. Whelan,

' His Attorneg.

Feb. 19, 1935. D. 6. WHELAN 1,992,021

VOLUME CONTROLSYSTEM Filed Sepfc. 15,1931 2 Sheets-Sheet 2 Inventor: Dow O. Whe Ian H is Attovneg.

Patented Feb. 19, 1935 UNITED STATES VOLUME "common srs'rmu Dow o. Whelan, lladdonfield, N. 1., assignor to General Electric Company, a corporation of New York 7 Application September 15, 1931. Serial No. 562,961 18 Claims. (0!. 2-50-20) My invention relates to signal receiving systems, particularly to means for controlling the intensity of the signal emitted by the loudspeaker or other signal indicating device of aradio'or other'signal receiving system, and has for its object the provision of'an improved volume control arrangement for such systems.

7 In the o eration of radio receivers it is-necesknown as fading, or as occur when tuning the receiving system from station to station and thereby causing sounds of undesirably loud volume to be produced due to signals from nearby, powerful stations. i V I I Manual control means have been employed for all of the above purposes but by reason of the inconvenience involved in the repeated 'readjustmerit of the volume control necessary to compensate for fading, and by reason of the dimcul ties encountered in so coordinating manually the v adjustment of the tuning control' 'and volume. control as to prevent undesirable sound volume when tuning from station to station, it is usual practice to provide automatic volume control means to maintain substantially constant: the

level or" sound volume determined manually by the operator. V

Various systems of volume control for the purposes above described have been proposed in the past. In certain of these systems means have been provided for varying the bias voltage impressed on the control electrodes of amplifier devices incorporated in the :receiVersyStem, or to vary the anode voltage of the amplifier devices, thereby varying correspondingly the intensity of the signal impulses transmitted to the loudspeaker. These and similarlmeans for volume control in radio receivers and the like have not been found altogether satisfactory, however, when employed as manual or other volume control means for causing extreme changes in the intensity of the sound emitted by the loudspeaker, by reason of the tendency to distortion of the signals transmitted through the electronic amplifiers utilized. as part of the volume control means. j 7 v V In accordance with my invention these-dimcultiesare overcome by the provislonin a radio or other signal receiving system of volume control means operating on the field flux of the 'reproducer unit or loudspeaker. This volume control means preferably comprises a leakage path or magnetic shunt forming an auxiliaryv gap in the loudspeaker magnet structure, and an auxiliary winding associated with the shunt to vary the flux in this auxiliary gap, the winding being connected to a unidirectional current circuit forming part of the signal receivingjsystem. V

In a modification of my invention the volume control means operating on the field flux of the. reproducer unit comprises means including a' main fleldi winding oi. thisun'it for varying the total field flux. In this modification the control of sound volume is accomplished by connecting the main field winding to a unidirectional current circuit and by shunting or reversing a portion of the field w1nding, or otherwise arranging the' winding to varythe field flux.

-'Ihe volume control system in accordance with my invention may be so arranged that'the intensity of the signal impulses impressed on the loudspeaker is varied in accordance with current changes in the unidirectional current circuit, to which the winding of the reproducer unit is connected, to compensate for the effects of fading or to prevent the excessively loud sound response above described; when tuning from station to station, the system thereby providing automatic volume control without distortion over a wide range the intensity of the signals impressed on the receiving system. The volume control system inaccordance with my invention may be arranged to provide also, in combination with an automatic volume control means, a semi-automatic volume control'in which the desired levels of sound signals which are maintained constant by the automatic means are obtained-manually by the above mentioned means operating on the field flux of the reproducer. Further, the volume control. system comprising the above mentioned means may be so arranged that the sensitivity 01.

ing the loudspeaker shown in Fig. 1 and including an automatic volume control arrangement in accordance with my invention; Fig. 3 is a sectional view of a loudspeaker of the electro-dynamic type embodying my invention; Fig. 4 is a circuit diagram of a receiving system incorporating the loudspeaker shown in Fig. 2 and including an automatic volume control and a sensitivity control in accordance with my invention; Fig. 5 is a circuit diagram of a radio receiving system including a semi-automatic volume control system having a manually operated volume control means in accordance with my invention combined with an automatic volume control means; and Figs. 6 and '7 are modifications of the system shown in Fi 5.

In Fig. 1 is shown a loudspeaker comprising an electromagnetic driver unit 10 including a magnet core 11 having field coils 12, 13 mounted thereon, and an armature 14 arranged to vibrate in a main air'gap 15 formed by a pair of polepieces 16 between which and surrounding the armature 14 is mountedanactuating or. voice coil 1'7. The armature 14 is connected to a diaphragm 18 by a driverod 19; In accordance with my invention, the electromagnetic driver 10 further includes a second pair of pole pieces 20 forming an auxiliary air gap 21, and on these latter pole pieces is mounted a differential or shunt coil 22 adapted to vary the flux'in the auxiliary flux path formed by the pole pieces 20 and air gap 21. r

The operation of a volume control system comprising the driving unit 10 willbe clear by reference to the circuit diagram of Fig. 2.

In Fig. 2. signals, of voice frequency areimpressed from any suitable source (not shown) on the control electrode 23 of an amplifier'including an 5 anode 25. Current for the anode circuit is supplied preferably-from an alternating current sourcejtnot shown) and rectified,- the resulting unidirectional current passing through a filter 26, certain'filter reactors of whichare preferably constituted by the field :coils 12, 13 ofgthe; loud speaker driver 10'shown in Fig. 1.

The output circuit ofamplifier 24 which-represents the audio frequency amplifier stage imme-.

diately precedingihe loudspeaker, or-the last audio frequency amplifier stage if more; than one stage are employed, is of the'type characterized by an outputcondenser and an outputchoke or output reactor. In the system illustrated in Fig. 2-the alternating component of'theamplifier. output is, therefore, impressed on the voice coil 17, represented diagrammatically-inFig. 2 and inperspective inFig. '1, through an output condenser 27 in theusualmanner. r .7

In accordance with my invention, however,the outputrchoke-or reactor through which the direct component of the amplifier anode current flows is constituted by the auxiliary winding 22 of magnet core 11, above described in connection with Fig. 1. This winding-shownin perspective in Fig. 1, is illustrated diagrammatically in Fig. 2 as connected between the anode and a voltage a divider resistor 28 connected to the filter 26.

Assuming that the intensity of signals impressed on the control electrode circuit of amplifier24 varies undesirably by reason of fading or by reason of tuning the receiver from station to station, the signal oscillations impressed on voice coil 17 of driver 10 from the output circuit of amplifier 24 tend to vary in amplitude, with a corresponding variation in the intensity of sound waves emitted by the loudspeaker. As the anode ctu'rentof amplif er 251; varies, however, changes pressed signals.

occur in the flux in the magnetic shunt or leakage path, constituted by the auxiliary pole pieces 20 and air gap 21, on which is mounted the auxiliary winding 22 constituting the output reactor of the amplifier. The winding 22 is so arranged that as the current therein increases, due to increase in the intensity of'the signals impressed on amplifier 2 4, the flux in the auxiliary air gap '21 increases correspondingly, a portion of the flux in the main air gap 15 being shunted therefrom through the auxiliary air gap 21. The consequent reduction in the flux density in main air gap 15 is accompanied by a reduction in the amplitude of vibration of armature 14 mounted in the main air gap. In the same manner a decrease in the current in auxiliary winding 22 is accompanied by 1 By the provision, therefore,-of the magnetic shunt or leakage pathand of the auxiliarywin'ding associated therewith and connected in the output circuit of amplifier 24, the volume of sound waves emitted by the loudspeaker tends to remain automatically at proper levels during periods of undesired variation in the intensity of signals impressed on the receivingsystem.

the embodiment of the invention illustrated in Figs. 3 ands, a loudspeaker of theele'ctroa dynamic type is shown in Fig. 3 including a usual magnet structure 29 having a main air gap 30 formed therein between a core 31 and a pole piece 32 having'a central aperture. A; diaphragm 33 is arranged-to be driven in the usual manner by a voice coil; 34 mounted to vibrate in the main air gap and afield coil 35 is mounted on the core 31.

In accordance with my invention a magnetic shunt or leakage pathis formed in the magnet structure by meansof an auxiliarypole piece 36 interposed between core 31 and pole piece 32 and connectionwith the embodiment of the invennected through an output transformer 41 to the voice coil 34, and auxiliary winding 38 of the speaker shown. in Fig. 3 is connected in series with'the output circuit 39 through a reversing switch 42. v Manual volume control means is-provided comprising a variable resistor 43 connected across the output circuit 39.

The operation of the form of the invention shown in Figs. 3 and 4 is in general the same as that of the form shown in Figs. 1 and 2, the auxiliary winding 38, through which flows the output current ofamplifier 40, operating to change the distribution of the fluxthrough the main air gap 30 and the auxiliary air ap 3f7, the system then functioning, as above described in connection with Figs. 1. and 2, as an automatic volume control to compensate for changes in the intensity of im-.

In case, however, the system shown in Figs. 3 and fl is used for the reception of signals from distantstations, necessitating the highest degree of sensitivity in the receivingsystemflhe direc-. tion of the output current of amplifier 40 through wind gfifi nam r er es?" err- P swit 42,-. whereby as the relatively weak signals are impressed onthe receiver adecrease instead of an increase or the flux in the auxiliary-gap 3'? cc:- curs and a corresponding increaseinthe-jintensity of sound emitted by the loudspeakergby reason of the increased flux inma'in air gap 39 in which is mounted voice coil- 34-. It will be evldent that means such as switch 42 for increasing the sensitivity'of the'loud'speaker' during re ception of weak signals may be incorporatedwithout diinculty inthe volume control system shown in Figs. 1' and 2. 7

"lheloud'speakers illustrated in Figs. 1' and 3 have been described as incorporating magnet cores having field coils mounted thereon. It will be understood, however; that the field structure or the loudspeakers may be of the permanent magnet type. In the embodiment of the invention illustrated diagrammatically in Fig. 5 the 'sign alreceiving system in which a semi-automatic volume control is incorporated is shown for purposes of illustration as a radiobroadcastreoeiving apparatus of the superheterodyne type with which is operatively' combined" a socket power unit Ior'supplying direct voltages to the electronic devices of the receiving apparatus from analternating current source. The receiverportion' of the system includes an antenna 4450011131811 toa radio fre quency amplifierAS, a 1st detector, an oscillator, two i'nterrnediateradio frequency amplifiers 46 and 47, a 2nd detectona power or output'ampliher, and a reproducer unit or loudspeaker 48 which is preferably of the. electrodynamic type.

The power unit portion of the receiving systemincludes a transformer, only a part of which, 49,

is shown in the figure, connected t'o-an alternating current source, rectifying tubes 50, a filter fil, voltage dividerresistors 52 and 53, andfilter 'reactors 54and55 one'of which as-55 is preferably constituted by a field coil of the-loudspeaker 48 similar to thefield coil 35 ot'the-reproducer unit shown in Fig. 3. The'above'apparatus illustrated in Fig. 5 is oi well knownconstruction andneed not be described further. a w

In order to enable the operator or the receiving system illustrated in Fig. 5 to obtain desired volume of sound from the reproducer unit'48', 'and, at the same time, inorder to maintain the-level oi volumeof sound at anypredetermined desired value over arange-ol variations in theintensity of the signals impressed onthe antenna 44 the semi -automatic volume control system which is provided comprises a manually operated control portion and an automatic'portion.

' Of this semi-automatic volume control system the automatic portion may be of any'well known form but comprises preferably an automatic-"vol ume control or pilot tube 56 having a cathode 57, an anode 58 and a control electrodenrgrld' 59. This tube maintains the predetermined level of sound volume constant in a usual manner by controlling the intensity ofthe transmitted signal at some point or points of the receiver precedl ing the output or power amplifier stage. In this form ofautomatic volume control the signal voltages impressed on the input circuit 60" of, the 2nd detector are also impressedthrough. acondenser 61 on the grid 59'of the pilot tube 56, therebyin fluencing the current in the anode circuit'of this 7 tube. Since the bias potential impressed on the control electrodes or gridsfihto or amplifiers to 47 is constituted by the voltagedrop across a resistor '65 inthe anodecincuitofi pilottubetfi a strong signalimpressedaon the grid 59101? thepilot action of the pilot tube. This tube, therefore, op-' crates to maintain the intensity of the signal im-- pressed onthe reproducing unit 48,- and the vol-- 'ume of sound emittedithere from, at a predeter+ mined} level;

In semiautomatic volume controlsystems em ployed heretofore incorporating a volume control or pilot-tube, astube 56' shownin Fig. 5,the manual adjustment of the receiver for a predetermined level of sound' volume has been accompli'shed by varying the bias voltage impressed on the grld-oithis pilot tube, for example by con necting its grid-through manually operated switch me'ansto-a continuously variable resistor connected to the filter of the alternating current supply means. In accordance with my inventioii', however, the pilot tube, associated as-a bove do? scribed with the grids ofthe amplifiers 45 to 47 and with the input circuit 60 of the 2nd detector, is relieved from-the work ofdetermining by man- Inthe embodiment of the invention illustrated inFig. 5'this part of the semi-automatic control of sound volume is accomplishedby means simi lar ingeneralflto the;voluine control system, in cluding means operating on the hurt inthemagnet system of the reproducerunit, above described in connection withtheembodirnents of the in vention illustrated in Figs. 1' to' 4L In the latter. volume control systems a winding; as coil 22 of Figsi'l' and 2, and coil38 of Figs. 3 and 4,is described as connected to a unidirectional current circuit of the receiving system, the current in whlch'varies in accordance with the variations in the intensity of the signals impressed on the system, the winding operating on the'field flux in a'manner to maintain predetermined levels of sound volume automatically.

The'volume control 'mean's'in the system illus-- trated in Fig. 5, however, including a winding, as

tube, ordinarily producing aloud sound'irom the reproducing unit 48; causesanincrease in anode ual*operation the desired'level ofsound volume-.

66, operating on the field flux of the reproducer current in that portion of the filter to which the Winding '66 is connected being substantially constant, or varying 'only to a slight degree with variations inthe intensityof signals impressed on the system of which the filter forms a part.

In order to vary manually the current flowing in winding 66 the latter is connected to a con-'- tinuously variable resistor'fi'l connected in the filter 51, adjustment of the" variable resistor, to vary the amount of filter load current flowing through coil 66, operating .to vary the effective flux in the magnet of the reproducer 48 and consequently the level ofthe sound volume. When this level .ha-s -jbeen established by the described manual adjustment; ofnthe filter load current flowing in coil 66, the level is then maintained constant by the action hereinbefore explained of the automatic portion of the semi-automatic control system, this latter portion comprising pilot tube 56.

It will benoted that the semi-automatic volume control system illustrated in Fig. 5 and above described has'the marked advantage over the usual semi-automatic system employing an automatic volume control tube or, pilot tube whose control electrode voltage is manually varied that a much wider control range is covered without distortion and without thenecessity of employing highly specialized tubes for the control of sound volume. To produce this beneficial result, in the system shown in Fig. 5 the manual control ofthe sound volumev level, which after being setmanually at a desired value is maintained constant by the pilot tube, is efiected by means operating not on the bias voltages of the amplifier tubes but only on the field fluxof the reproducer unit, thus permitting all of these amplifier tubes, controlled by the pilot tube, to operate on that bias voltage which produces maximum quality of sound reproduction. This higher quality of reproduction is attained, by this means, even when the pilot tube operates over narrow limits of volume variation.

In the above description of operation of the volumecontrol systems illustrated in Figs. 1 to 5, Ihave set forth that the control of the field flux in the reproducer unit is accomplished by means comprising a magnetic shunt and an auxiliary field winding cooperating therewith to vary the 1 distribution of flux between the main air gap and an auxiliary air gap. "It will be readily; understoodhowever that instead of employing a magnetic shunt and an auxiliary winding to vary the field flux the total field flux may becontrolled, and the intensity of sound emitted by the reproducer unit correspondingly varied, by connecting a main field coil or coils, as field coils l2 andf35 of Figs. 1 and 3 respectively, to a unidirectional current circuit forming part of the signal receiving system as shown for examplein'Figs. 2 and 5, and by providing suitable switch means, as required for manual control of the sound volume level, to connect the field coil in any .suitable manner to the unidirectional current circuit, for example toshunt or reverse a portion of the coil,

. thereby varying the total field flux in the main air, gap in which the vibratingarmature l4-of Fig. 1 or the voice coil 39 of Fig. 3 is mounted.

Since in the semi-automatic volume controlsystem illustrated in Fig. 5 the-control or pilot tube 56 is not operated as a means for manual adjustment of the level of sound-volume but only for automatic maintenance, of this level, the grid circuit of this tube does not require the continuously variable resistor means provided heretofore for effecting the manual adjustment. 7 Therefore, in the form of the invention illustrated in Fig. 5 the grid 59 of pilot tube 56 is connected to-the voltage divider resistor 53 by means of a tapped switch 69, whereby preliminary adjustment of the automatic volume. control or pilot tube is accomplished by simple switchmeans, this form of switch permitting the use ofrelativeiy high resistance values between adjacent control points of the switch, not obtainable in a switch means comprising a volume control resistor :which is continuouslyvariable. V

In order to provide for the larger variations in the sensitivity of thereproducing unit 48 the field windings of the reproducerunit may be C011.

nected to,.the current source'through tapped switches, as switch 701connected tothe field wind: ing 66, or the field windings maybe arranged forseries parallel tappingfor this purpose. It I will be readily understood, further, that for the purpose of increasing the sensitivity ofthe reproducer when receiving weak signals, means may be provided for reversing the direction .of current through the field winding 66 in the same manner as described in connection with thewinding 36 andswitch 42 shown in-Fig. 3..

The modification of my invention illustrated inFig. 6 is all respects the same as that illustratedin Fig. ,5 except that, whereas in Fig. 5 the main field winding 55 and auxiliary winding 66 of reproducer unit, 48 are-connected inthe same section '71 of filter 51, in Fig. 6 these wind-. ings are connected each in a different section 01 a'filter. In Fig. 6 main winding 55 is connected in; one section '72 of the filter 73, across a field protective short-circuiting switch '74 asin Fig. 5, and auxiliary winding 66 is variably'connected to the resistor 67 in another section of the filter 73.

'I'he modification of my invention illustrated in Fig. '7, is similar in constructionand operation to the modifications illustrated in Figs. 5 and 6. The modification shown in Fig. 7, however, differs from those of Figs. 5 and-6 in that at least one of the field windings 55,66 of thereproducer unit 48 is connected to the current source filter in parallel with the load. Whereas'in the modifications shown in Figs. 5 and 6 both windings are in series with the load connected to the filter, in Fig. '7 the auxiliary winding 66 only is connected in series with the load in filter 76, the main winding 55 being connected across the filter in parallel with the load therein. Whatl claim as new and desire tosecure by Letters Patent of the United States,,is:'-- 1 1. The combination with a signal systemcom prising a unidirectional current circuit and a signal indicating device including means providing a magnetic circuit having an air gap, a, movable element mounted in saidgap,and'ineans to actuate said element in accordance with signal current in said system, of means to provide distortionle'ss. control. over a wide frequency range of the intensity of the signal produced by said signal indicating device, .said means includinga winding connected to said unidirectionalcircuit and associated with said magnetic circuit to vary' means remaining fixed in position with relation to said magnet during variation of the intensity 0! said signalsn V 3. Inja signalreceivingsystem,-an input circuit, a unidirectional current circuit, the current in said unidirectional current circuitvarying in accordance with the intensity. of signalsimpressed on said-input circuit, a'sound producing device including a magnet having; an angst and an element adapted to Vibrate therein, {and I'nrean's to provide distortionless control over; a Wide ire quency rangeof the volume of sound eii'iitted by said sound producing device, said means includ ing a winding connected to said unidirectional current circuit and associatedjwith said magn t to vary automatically in accordance withthe intensity of said signals impressed on said input circuit the intensity of sound Wave'sfpio'difod b'y said sound device, said means reinaihing fixed in position with relation to saidmagnet during vairiation of the intensity ofs'aid' signals.

4. In a signal receiving system, an em Cult, a'; unidirectional current cifcfiit,'thecufreht in said unidirectional circuit" varying accordance withthe intensity of signals impiessed on said input circuit, a sound precise g device incl'uding a magnet having n air gap and al'vilbiat ing element mounted therein, means providing a leakage path'for thfe ir'r saidflagfiet, and means to provi'de' distorti'cnless commr over a wide frequency range of the meme unsound emnfesmy said sound producing device,'"sai 'd mea s including a winding ccnmcteu to said unidirectional current circuit and associated with said magnet to vary automatically thei'fiu'xf in said leakage path'in a predeter fiiiied manner in accordancetwith the current infsai'd' unidirec tional current circuit, said niea'nsproviding a leakage path remaining fixed' iii'bosit'ib'ri with relation to said magnet durin Variationof the flux in said 1eakage path wnereby the intensity of wave emitted by Said spun- 1' device i' varied in accord nce with theii'ltenjsi'fiy ef said impressed signals: V H 5 a's'ignal recei g systeiii, amnput air edit, ascend producing de iceimmdifiea ma net having an air ga yam a vibratingieiement mounted therein, and means to provide distor' t'icnless control over a; widenequency rarng e of the vomme of sound emitted; y; said soun emitting device} said fneahs'i'r'icludi'i'ig' winding associates withsaid ma net and iiifvvhiclf flaws current va ying iii' accor nce. with the tensity of signals impressed on said nput circuitto" va'ry automaticanymenux n aid magnet, s I mfg beingso" arranged 12 at cn inten of the sound Waves; emitted by said? sound device varies automatically inversely as th ntensity of' said impressed signals, whereby the mtensityfofsai'd emitted sound waves tends. to} rema n substan many constant' witn v rying ntens tydf said impressedsignals, said means manning said winding remaining fixedinposition with relation to said magnet during venetian 01" the flux in said magnet: H V a j s. In a signal l eeeivm's system; an amuse? including an input circuit and an outbutbircuit; a; sound producingQdevice operatively assoc ed with said amplifier ana mcmdmg a rna'g v having an" air gab and a; vibrating element mounted thereirijand me ns" to provide disltof? tionlessic'ontrolf over awide frequencyja g'eof the v mme ofsound emitted by 'sai saunaerfiittingdevice, sfaid means'includin a winding onneeted to 1 said output circuit tolvary" the flux in s'aid magnet, whereby the intensity" of sound waveemittedby said sounddev'icetendsjto'vary utcmatic'anyi in accordance with the intensity oflsig'n'als' mpressed: (in Said i'nfiut ciic' said means'i'nciud ng saidflwinding'rehiairiiiig axes in positibn witnieiation to V id magnet during; varistibnzoithefiux'm' mag-net; g "7: Iri' a signal receiving system; an" ampnfi'er' fixed length during varia V '5 mending an input circuit and sn'aiitbut' eirifit,

asc nd producing dev ce adapted to be operated by said amplifier and including a magnet ,ha igg an air'gap and a vibrating elemestmoumed thereinmeans to 'pre'viae aistoftionlessvcontrol over wide frequency range o t e emitted bysaid sound emitting including a winding connected volume of sauna device, said r'n'ans to said output cirrrieans operative l'elaticinwitn said winding to cause' said flux to changin' a predetermined inann r, whereby the intensity f sound waves efliittd by said stan device tends to infease in (me position or saidsfi itch and to diminish ariouiei lbosi'tiojn thereof accordance with trieiintnsit'y quency rangeortn intensity 6f the sihlljifdemcee by sa i'dTsighal indicating" device, said neans Iiiovid'i'ng magnetic leakage bath in said; netic circuit, anume'sns including a winding 1: 1; ne' ted to-said unidirectional current circuit and "akage fiath remaining fixed in positieij; viith r'etioh to said signal indicating deviceduring 'tioncftnnuiinsaidgn; I591 'Ifhe Gentleman" with an electricsy'stem com ris ng a unidirectional current circuit and v. gm m V ma n t im a. an? .aac1 an em n adapted to vibrate in said gap, of meansto provi'd distrtioiiless control over a wide frequency range 0' thein nsi'tyof the s iginal produedby s' id signal l lf l dlCat ifig device, s'aiduneans includauxiliary ngels in sai V V and in ansfiiicliidihg' a winding connected to said unis ti ai current circuit n assoeiated with sa d auxiliary ,air gap to var the flux in said Iiiaih airga'xi', said auxiliary; air gap remaining of tion of the flux in said mainaireaii l c"oi'ii l5i najt ioiii vv ith-an electric system structuiehav ng I sentient vibrate in said gap of rneans to providedistbrtioiiles's cofitrfjl over a w ideijrequency essay of the s gnal produced by I sald signal indicating device, said means includ:

iii'gf'alnauiiiliary airg ap in said-magnet structure;

and means including a winding connected to said an gapjreinaining of figzed length durn 6: the a rnount of fiux shifted from tlie inain air'gap tothe auxiliary air gap;

11 al efi e ing-s st m; an am li e in'cludihg input and gutput circuits,;a source pf um vice' airar'igfe be opera e I by said a mplifier; and including magnet structdiehaying; a n air gas an a. vibial g? ele vii ijo'unted tlnerein, andni'e' 'e'distortioxiless control over avv'ijde'freciii'eniiyr said sauna attaching de said mean cluding a magnetic shunt'iiisaid magnet struc- :'e a: messengamnesty to vary the flux in said-magnet and svv i tch 7 dimaghet structure,

a amab adia i i mea ture and meansincluding a winding associated with said shunt to vary the fiux therein said source of unidirectional current being connected to said output circuit through said winding, said magnetic shunt remaining fixed in position with relation to said magnet structure during variation of the flux in said shunt.

12. In a signal receiving system, an amplifier including an input circuit and an output circuit, a source of unidirectional current, a sound producing device including a magnet structure havingan air gap and a vibrating element mounted therein, means including a capacitance to connect said sound device to said output circuit, and means to provide distortionless control over a wide frequency range of the sound emitted by said sound producing device, said last-named means including a magnetic shunt in said magnet structure and means including a winding associated with said magnetic shunt to vary the flux therein, said winding being connected between said current source and said output circuit and constituting the output choke of said amplifien'said magnetic shunt remaining fixed in position with relation to said magnet structure during variation of the flux in said shunt. 1

13. The combination with' a signal system adapted to receive signals of varying intensity and comprising an amplifier of frequencies above audibility, a unidirectional current circuit, and a sound producing device including a field magnet having poles forming an air gap and a vibratable member mounted therein, of means to maintain the intensity of sound waves emitted by said sound device at predetermined levels, said means comprising volume control means connected with said amplifier and operating in accordance withcurrent therein to maintain automatically the intensity of signals impressed on said sound device substantially constant during periods of variation in the intensity of signals received by said system, and means to control manually without distortion over a wide frequency range the intensity of said emitted sound waves including a winding connected to said unidirectional 'circult and arranged to vary the flux in said air gap, said last-named means remaining fixed in position with relation to said field magnet during variation of the flux in said air gap.

14. I'he combination with a signal system adapted to receive signals of varying intensity and comprising an amplifier of frequencies above audibility having a control electrode, a detector having an input circuit, a sound producing device including a field magnet having poles forming an air gap and a member adapted to vibrate therein, and a unidirectional current circuit, of means to maintain the intensity of sound waves emitted by said sound device at predetermined levels, said means comprising an electronic device arranged to vary the bias voltage of said control electrode of said amplifier in accordance with the voltage in said input circuit of said detector to maintain automatically the intensity of of signals impressed on said sound device substantially constant during periods of variation in the intensity of signals received by said system, and means to control manually without distortion over a wide frequency range the intensity of said emitted sound waves including a winding connected to said unidirectional circuit and arranged to vary the flux in said air gap, said lastnamed means remaining fixed in position with relation to said field magnet during variation of the flux in said air gap.

15. The combination with a signal receiving system comprising an amplifier device having a plurality of electrodes, a detector, a sound producing device including a field magnet, means to impress signals on said sound device from said detector, andmeans including a current source and'afilter'for supplyingcur'rentto the output circuits of said amplifier and detector, of means to control the intensity of soundwaves emitted by said sound producing device, said last-named means comprising volume control means connected with one of said amplifier electrodes and with the input circuit of said detector and operating in accordance with current in said amplifier to maintain automatically at a predetermined value the signal energy transmitted from said amplifier to said detector, and means to vary manually independently of .said automatic-means the intensity of said emitted sound waves without distortion'over a wide frequency range comprising a winding connected to said filter and constituting a filter reactor therein, said'winding being associated with said field magnet for varying the flux therein, said manually variable means remaining fixed in positionwith relation to said field magnet during variation of the flux insaid magnet. q t a 16. The combination with a signal receiving system comprising a plurality of amplifiers each having a control electrode and an output circuit, a detector having input and output circuits, a loudspeaker including a field magnet, means to impress signals on said loudspeaker from said detector output circuit, and means including a current source and a filter iorsupplymg current to said output circuits, of means to control semiautomatically the intensity of sound waves emitted by said loudspeaker, saidlast-named means comprising an electronic device to vary automatically the bias voltage of said control electrode in accordance with the'voltage in said detector input'circuit, and means to vary manually without distortion over a wide frequency range the intensity of said emitted sound waves comprising a magnetic shunt ins'aid field magnet and a winding associated with said shunt for varying the flux in said field magnet and constituting a filter reactor ofsaid filter, said manually variable means remaining fixed in position with relation to said field magnet duringvariation of the flux in said magnet; 17. In a signal receiving system, a plurality of amplifiers each having a control electrode and an output circuit, a detector having input and output circuits, a loudspeaker including a field magnet, means to impress signals on said loudspeaker from said detector output circuit, means including a current source; and a filter for supplying current to said output circuits, and volume control means for saidsys'tem comprising means including an electronic device having a control electrode to vary automaticallyfthe bias voltage of said control electrodes of saidamplifiers in accordance with the voltage in said detector in put circuit, means including a tappe'dresistor associated with said filter for supplying'potentials to saidcontrol electrode of said electronic device, and means to vary manually without distorttion over a wide frequency range the intensity of sound waves emitted by said loudspeaker including means providing "a magnetic shunt in said magnet and a tapped winding associated with said shunt and constituting a'choke coil to'vary the flux in said shunt; said means providing a magnetic shunt remaining fixed in position with 1,992,021 relation to said field magnet during variation of the flux in said shunt.

18. In a signal receiving system, a plurality of amplifiers each having a control electrode and an output circuit, a detector having input and output circuits, a sound producing device including a field magnet having poles forming an air gap and a vibratable member, mounted therein, means including a current source and a filter for supplying current to said output circuits, and volume control means for said system comprising means including an electronic device having a control electrode to vary automatically the bias voltage of said control electrodes of said amplifiers in accordance with the voltage in said detector input circuit, means including a resistor associated with said filter for supplying potentials to said control electrode of said electronic device,

which means including taps in said resistor adapted to be connected to said electrode for preliminary adjustment of said electronic device, means to control manuallyrwithout distortion over a wide frequency range, the intensity of sound waves emitted by said sound device including a winding associated with said field magnet and arranged to vary the flux in said air gap, said last-named means remaining fixed in position with relation to said field magnet during variation of the flux in said air gap, and means to supply current to said winding including a continuously variable resistor connected to said filter for fine adjustment of the current in said winding and switch means comprising taps in said winding for preliminary coarse adjustment of said current in said Winding.

1 DOW O. WHELAN. 

